The invention relates generally to a bearing, and in particular to a compliant gas bearing that uses integral wire mesh dampers.
High speed equipment, such as aircraft engine turbomachinery, and aero-derivative applications, such as steam turbines, gas turbines and compressors, must pass through several natural frequencies or critical speeds before reaching the design operating speed. When a system operates at its natural frequency or critical speed, the system/rotor vibration amplitudes become large. These vibrations, stemming from rotor imbalance can be destructive or even catastrophic if not adequately dampened. Bearings with adequate damping characteristics limit or damp out the synchronous vibrations to allow the equipment to safely pass through the critical speeds. Furthermore, good bearing damping contributes to rotor dynamic stability at high speeds by suppressing subsynchronous excitation of rotor natural frequencies.
Fluid or oil film journal bearings have long been used to dampen the vibration created by turbomachines. Rotors in aircraft gas turbine engines and industrial centrifugal compressors often use oil-based squeeze film damper bearings supported by squirrel cage centering springs to control vibration amplitudes. In a fluid film bearing, a thin fluid film forms a buffer between the rotating journal surface and the stationary bearing surface, and dampens vibration from the rotor. In a squeeze film damper bearing, a thin film of fluid in the form of a liquid, usually oil, is squeezed by two non-rotating cylindrical surfaces. One surface is stationary while the other is positioned by a centering spring structure and orbits with the motion of the rotor. The squeezing of the fluid film in results to the orbital rotor motion dampens rotor vibration through the bearing support.
The simplest squeeze film damper bearing design does not include a centering spring. The outer race of a rolling element bearing, or the outer bearing shell in the case of a fluid film bearing, is allowed to float and presses in a clearance space between the bearing outer diameter and the housing inner diameter. The absence of a mechanical centering spring in this design configuration means that the damper journal will be bottomed out at start-up. As the speed increases and the shaft starts to rotate, the damper's journal (bearing shell outer surface) will lift off. The oil film in a squeeze film damper does not produce direct stiffness like conventional fluid film bearings. However the damper does develop direct stiffness-like behavior. This direct stiffness is due to the cross-coupled damping coefficients, which exhibit stiffness-like (spring) characteristics.
The non-centered damper is one of the most non-linear of the squeeze film damper designs. There are two basic mechanisms that are responsible for this non-linear behavior. The first of the two non-linear mechanisms is attributed to the non-linear characteristics produced by the cross-coupled damping coefficients. The second source of non-linear behavior present with this type of damper comes as a direct consequence of the bottoming out of the damper journal.
The simplest means of providing a centering spring in a squeeze film damper is through the use of elastomer O-rings. The advantages of this design stem from its simplicity, ease of manufacture, and the ability to incorporate the damper into small envelopes. Some of the disadvantages with this design are attributed to the limited range of stiffness that can be achieved with elastomers. Predicting the stiffness with a good degree of certainty is difficult in elastomeric materials due to the material variance, and the influence of temperature and time on its properties. The O-ring design is also susceptible to creep, causing the damper to bottom out, which, as discussed above, may lead to a bi-linear spring behavior.
The most common used squeeze film damper design, particularly in aircraft engines, is the squirrel cage supported damper. A distinctive feature necessary with such a design is the relatively large axial space required in comparison to the damper length. This is one of the major drawbacks of this damper design. The squirrel cage forming the centering spring for the damper quite often requires three to four times as much axial space as the damper itself.
Assembling the squirrel cage spring and centering the journal within the clearance space requires special tools and skills. The squirrel cage spring also complicates the damper end seal design and assembly. It is also very difficult to offset the spring assembly, in order to account for the gravity load due to the shaft weight. Maintaining parallelism between the damper journal and housing is another factor that adds uncertainty and complications to this design.
Another oil-based squeeze film damper design includes an integral damper centering spring. In this design, cantilevered support ribs, along with the sector they are supporting at both ends, form a centering spring element. A small gap between the sector and the outer ring forms the squeeze film damper clearance space. Unlike the squirrel cage spring design, the integral damper centering spring design does not occupy any additional axial space beyond the existing length occupied by the bearing. The complete assembly may contain any number of sectors, depending on the load and required stiffness and damping for the particular application. Wire electric discharge machines (EDM) provide an excellent means of obtaining the desired clearance with very high precision and repeatability maintaining excellent parallelism between the damper journal and housing.
Despite the advantages offered by these squeeze film bearing constructions, oil lubricated bearings impose a much higher cost and maintenance load as well as reliability problems associated with oil leakage, filtering and conduits. These and other drawbacks of oil lubricated bearings have led industry to seek to develop a compliant air foil bearing. However, present day air foil technology has been primarily limited to small lightweight rotors and machines, namely air cycle machines (ACM) on aircraft. Thus, while the benefits of incorporating air bearings into on aircraft are well understood, significant technical challenges have to be overcome to develop a practical design. These challenges include developing a gas bearing with the load capacity and damping that is significantly higher than the present day air foil bearing technology.